Vacuum fluorescent displays (VFDs) are well known in the art as emissive, digital display modules, operating at low voltage. One of the drawbacks to the VFD, is the lack of color, and matrix addressability. As a result, the cathode ray tube (CRT) continues to function as the emissive display of choice. However, the CRT needs to operate at high voltage and is not flat. Recently, the Field Emission Display (FED) has been developed as a flat panel vacuum emissive display technology. The goal for a low voltage FED relies on the development of high efficient low voltage phosphors.
Vacuum emissive displays, such as cathode ray tubes, include an electron source which supplies electrons subjected to an acceleration voltage on the order of several ten kilovolts. Typically, these high energy electrons traverse a layer of conductive material disposed on a light-emitting phosphor which is formed on the inner surface of a display faceplate. The electrons thereafter impinge upon the light-emitting phosphor which is thereby excited to emit light. The light-emitting phosphors characteristically have poor electrical conductivity, and any negative charge that accumulates on the surface of the light-emitting phosphor, is bled off by the layer of conductive material. The efficiency with which electrons are transmitted through the charge bleed-off layer, is poor, resulting in at least a 1000 volt drop in electron acceleration potential. This results in lower brightness for the display. The trade-off in brightness between an aluminized phosphor screen, and a non-aluminized screen occurs at about 4000 volts. The efficiency of prior art phosphors at voltages below 4000 volts is marginal. Also, most prior art phosphors do not conduct sufficiently to remove the surface charge build-up. Thus, such a configuration, which includes a charge bleed-off layer, is not practical for use in high brightness VFD's or FED's operating at voltages less than 4000 volts. Much interest exists in developing FED's capable of utilizing electrons having lower energies, on the order of several hundreds to a few thousands of volts.
The use of low energy electrons provides many significant benefits and improvements over the high-energy electron precursors. The most significant is the reduced driver circuit costs of low voltage drivers. This is a significant cost for the final display. Two reasons not to use existing low voltage phosphors are the poor chromaticity for red, blue or green phosphors; and, except for ZnO:Zn, poor efficiency.
Independent of high or low voltage electron acceleration is phosphor degradation. When an electron strikes the phosphor surface, the electrons stimulate a catalytic reaction with adsorbed surface gases and the phosphor itself. The higher the electron current at the phosphor surface, the greater the reaction. The reaction products are released into the vacuum of the display, the species being capable of degrading or poisoning the electron source, such as field emission tips or filaments. Such degradation has obvious deleterious effects on the image quality of the display. In particular, the luminescent materials that are most efficient contain sulfur whose byproducts reacts with cathodes. These sulfur containing phosphors provide the correct color emission to create a CRT-like emissive display. Non-sulfur containing color phosphors are available, but are of poor efficiency when excited by electrons; or, they are of extremely low efficiency making them undesirable choices. Low efficiency phosphors require higher electron current to yield sufficient brightness. High current means more contamination to the emitters, and more heat in the display.
Initially, the development of VFDs and FEDs using low energy electrons was made possible because ZnO:Zn was an available phosphor known to emit light of blue-green color upon excitation with low energy electrons. In contrast to the high voltage phosphors, which have resistivities on the order of 10.sup.6 to 10.sup.12 ohm.cm, the resistivity of ZnO:Zn phosphor is 10.sup.3 to 10.sup.4 ohm.cm. At the lower resistivity, the ZnO:Zn phosphor can conduct away any charge build-up that occurs during excitation. This makes the ZnO:Zn phosphor a good material of choice for low voltage displays. However, because ZnO:Zn was the only known useful phosphor for low energy VFDs, the display color was limited to green.
Because the desire is to reduce power requirement costs, there has been an increasing demand for low energy VFDs and FEDs which provide images having multiple colors. Several approaches have been taken to provide phosphors which emit a variety of colors upon excitation by low energy electrons. In one approach, a phosphor having a lowered resistivity is provided by adding an electrically conductive material, such as indium oxide, to a high-resistance, color-emitting phosphor. This approach reduces resistivity to about 10.sup.4 to 10.sup.7 ohm.cm. The luminance of these lowered-resistance phosphors is only 15 to 45 per cent that of the ZnO:Zn phosphor being exposed to the same voltage conditions.
Another prior art scheme for providing color phosphors for low energy VFDs includes providing a ZnO.Ga.sub.2 0.sub.3 :Cd, or cadmium-doped zinc gallate material, which is physically mixed with a powder form of a high-resistance, color-emitting phosphor, that is excited very efficiently by ultra violet rays (UV). There are many known lamp phosphors that emit the proper color and are efficient when excited with UV. Because the gallate matrix is electrically conductive, the mixture does not charge up under electron excitation. Because the cadmium doped zinc gallate efficiently emits ultraviolet (UV) radiation upon excitation by electrons, the UV excitable phosphor emits with the correct color of visible light. The gallate-containing phosphor still does not exhibit the luminance of the constituent high-resistance, color-emitting phosphor when excited by high-energy electrons. The cadmium-doped zinc gallate material occupies volume in the final phosphor while not providing visible luminance, thereby providing reduced efficiency.
As will be described in greater detail below, the cadmium-doped zinc gallate material must be formed prior to combination with the high-resistance, light-emitting phosphor, because this prior art process requires treatments at temperatures upwards of 1300.degree. C. Many of the known high-resistance, light-emitting phosphors cannot withstand such high temperatures without chemically reacting with the UV emitting phosphor to form a new low emissive compound, or without decomposing to form a new low emissive compound. Thus, formation of the UV-emitting material cannot be carried out in the presence of the high-resistance, light-emitting phosphor.
In summary, admixing of a UV emitting phosphor with UV excitable phosphor is no more acceptable than admixing a high resistance phosphor with a conductive material like indium oxide. To reiterate, the functional relationship between the UV-emitting material and the light-emitting phosphor, they must be admixed together physically in the form of powders. In such a manner, particles of the UV-emitting material are dispersed between particles of the light-emitting material. The drawbacks of this physical configuration include reduced brightness and poor electrical conductivity because the particles of the cadmium-doped zinc gallate material, while having higher electrical conductivities than the light-emitting phosphor, are separated from one another by the particles of the light-emitting phosphor, which have poor electrical conductivities.
As mentioned above, the UV-emitting phosphor is included because, for many prior art phosphors, UV excitation provides more efficient light emission than electron excitation. Many prior art phosphors have a 90-100% quantum efficiency; that is, for each photon of UV radiation that excites the light-emitting phosphor, one photon of visible light radiation is emitted by the light-emitting phosphor. In contrast, electron excitation of these phosphors exhibits an efficiency between 1-20%. A 1% electron excitation efficiency means that for every 100 electrons received by the light-emitting phosphor, only one photon of visible light is produced.
The above prior art method of providing a low voltage phosphor by combining a UV-emitting material with a UV-excitable, light-emitting prior art phosphor suffers several drawbacks. When the low voltage phosphor is provided by mixing the UV-emitting material with the UV-excitable, light-emitting phosphor, not all of the electrons received by the mixture are converted to UV rays, resulting in the low efficiency emission referred to above.
Accordingly, there exists a need for a new and improved phosphor having improved efficiency and luminance under low energy electron conditions. There also exits a need for a new and improved phosphor which reduces the deleterious effects of outgassing which results in poisoning of the cathode.
The method for making the prior art gallate phosphor described above includes, first, mixing ZnO, Ga.sub.2 O.sub.3, CdCO.sub.3, and, in some instances, a rare earth being selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, and Tm, in appropriate amounts. The rare earth element, if included, emits a characteristic colored light, thereby providing the color-emitting function of the resulting phosphor. The prior art discloses the use of rare earth elements in low voltage phosphors solely for providing colored light; the Cd was found to enhance the UV emission of the zinc gallate phosphor. This mixture is then heated in an air atmosphere, to temperatures of about 1000.degree.-1300.degree. C. for about 5 hours. The rare-earth-containing phosphors resulting from this prior art method, however, have poor luminescence. If a rare earth is not included in the above mixing step, the UV emitter thereby produced is physically mixed with a color-emitting high-resistance phosphor to provide the prior art low voltage phosphor. Because many prior art phosphors are adversely affected by high temperature treatments, and because this method of making the UV-emitting substance includes firing at temperatures well above the temperature tolerances of many color-emitting phosphors, the UV-emitting substance cannot be formed onto the particles of the color-emitting phosphor via this prior art method. Rather, the UV-emitting, electron-excitable material must be formed in a separate step and thereafter physically mixed into the color-emitting phosphor, a process which limits the efficiency of the low voltage phosphor resulting therefrom.
Accordingly, there exists a need for an improved method for making a phosphor which can be performed at temperatures beneath the temperature tolerances of prior art, color-emitting phosphors.
Sol-gel technology, whereby a solution or "sol" becomes dense like a glass, is known in the art and includes methods for the formation of coatings, or thin films, on glass, ceramic, glass or ceramic fibers, and specialty shapes. Sol-gel processes include a myriad of chemistries, but have similar constituent steps, including gelation by, for example, hydrolysis of metal-organics in solution to provide a gel, drying of the gel, pyrolysis, and densification which results in crystallization of the material. Sol-gel techniques are favored for their low-temperature treatments. These temperatures (about 300.degree.-1000.degree. C.) are beneath the tolerance temperatures of most substrates used for display face plates, and beneath the tolerance temperatures of most light-emitting, high-resistivity phosphors.